The addition of coadsorbents and the introduction of electron-withdrawing groups in dye sensitizers are considered feasible strategies for improving the power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs). However, facile and precise predictions of the influence of these two strategies on their photovoltaic properties, including PCE, are challenging. In this contribution, we studied a known D-A-π-A indoline dye WS-2 adsorbed on a TiO2 anode represented by a supercell model. The PCE of this dye was evaluated to be between 9.69% and 13.70%, depending on the supercell representation, compared with an experimental value of 8.55%. The PCE could be increased to 16.39% on a moderate supercell by adding chenodeoxycholic acid (CDCA) as a coadsorbent. Such an enhancement could be ascribed to the intermolecular interaction between WS-2 and CDCA, suppressing excessively high dye coverage and thereby resulting in a remarkable increase in the open-circuit voltage. Based on WS-2, a new molecule WS-2a was rationally designed by substituting the benzothiadiazole moiety of WS-2 with a stronger electron-withdrawing thienyl-diketopyrrolopyrrole group. Consequently, the maximum absorption band showed a large red-shift from 522 to 638 nm, broadening the spectral response into the near-infrared region. A higher PCE of 16.62% was obtained for WS-2a. Moreover, the coadsorption of WS-2a and CDCA onto the TiO2 supercell achieved the best photovoltaic efficiency with a value as high as 24.15%. Therefore, the present study quantitatively reveals the impact of the coadsorbent and electron-withdrawing groups on the optoelectronic properties of dyes, which opens a new avenue to design high-efficiency D-A-π-A-structured organic sensitizers for promising DSSC applications. A discussion on the qualification of these results is given.